chore(mise): update configs and add cleanup script documentation

Update tooling configs, shell settings, and editor configurations
as part of the mise migration. Add companion documentation for the
cleanup-old-version-managers script and remove unused fish completions.

local/bin/dfm

@@ -169,7 +169,7 @@ section_install()
       if ! command -v mise &> /dev/null; then
         msgr nested "Installing mise..."
         curl https://mise.run | sh
-        export PATH="$HOME/.local/bin:$PATH"
+        export PATH="${XDG_BIN_HOME:-$HOME/.local/bin}:$PATH"
       fi
       mise install --yes
       mise reshim